This section is from the book "A Manual Of Photography", by Robert Hunt. Also available from Amazon: A Manual of Photography.
The following very remarkable process was communicated by Sir John Herschel, at the meeting of the British Association at York. The process cannot be regarded as perfect, but from its beauty when success is obtained, and the curious nature of all its phenomena, it is deemed important to include it, in the hope of inducing some investigator to take it up.
Sir John Herschel says, alluding to the processes just described, " I had hoped to have perfected this process so far as to have reduced it to a definite statement of manipulations which would insure success. But, capricious as photographic processes notoriously are, this has proved so, even beyond the ordinary measure of such caprice. * * * Paper proper for producing an amphitype picture may be prepared either with the ferro-tartrate or the ferro-citrate of the protoxide or the peroxide of mercury, or of the protoxide of lead, by using creams of these salts, or by successive applications of the nitrates of the respective oxides, singly or in mixture, to the paper, alternating with solutions of the ammonio-tartrate or ammonio-citrate of iron, the latter solution being last applied, and in more or less excess. * * * Paper so prepared and dried takes a negative picture, in time varying from half an hour to five or six hours, according to the intensity of the light; and the impression produced varies in apparent force from a feint and hardly perceptible picture to one of the highest conceivable fulness and richness both of tint and detail, the colour in this case being a superb velvety brown. This extreme richness of effect is not produced except lead be present either in the ingredients used, or in the paper itself. It is not, as I originally supposed, due to the presence of free tartaric acid. The pictures in this state are not permanent. They fade in the dark, though with very different degrees of rapidity, some (especially if free tartaric or citric acid be present) in a few days; while others remain for weeks unimpaired, and require whole years for their total obliteration. But, though entirely faded out in appearance, the picture is only rendered dormant, and may be restored, changing its character from negative to positive, and its colour from brown to black (in the shadows) by the following process:— A bath being prepared by pouring a small quantity of solution of per-nitrate of mercury into a large quantity of water, and letting the sub-nitrated precipitate subside, the picture must be immersed in it (carefully and repeatedly clearing off the air-bubbles) and allowed to remain till the picture (if anywhere visible) is entirely destroyed, or if faded, till it is judged sufficient from previous experience; a term which is often marked by the appearance of a feeble positive picture of a bright yellow hue on the pale yellow ground of the paper. A long time (several weeks) is often required for this, but heat accelerates the action, and it is often complete in a few hours. In this state the picture is to be very thoroughly rinsed and soaked in pure warm water, and then dried. It is then to be well ironed with a smooth iron, heated so as barely not to injure the paper, placing it, for better security against scorching, between smooth clean papers. If, then, the process have been successful, a perfectly black positive picture is at once developed. At first it most commonly happens that the whole picture is sooty or dingy to such a degree that it is condemned as spoiled, but on keeping it between the leaves of a book, especially in a moist atmosphere, by extremely slow degrees this dinginess disappears, and the picture disengages itself with continually increasing sharpness and clearness, and acquires the exact effect of a copperplate engraving on a paper more or less tinted with pale yellow.
" I ought to observe, that the best and most uniform specimens which I have procured have been on paper previously washed with certain preparations of uric acid, which is a very remarkable and powerful photographic element. The intensity of the original negative picture is no criterion of what may be expected in the positive. It is from the production, by one and the same action of the light, of either a positive or a negative picture, according to the subsequent manipulations, that I have designated the process thus generally sketched out, by the term "Amphitype;" a name suggested by Mr. Talbot, to whom I communicated this singular result; and to this process or class of processes (which I cannot doubt when pursued will lead to some very beautiful results) I propose to restrict the name in question, though it applies even more appropriately to the following exceedingly curious and remarkable one in which silver is concerned. At the last meeting I announced a mode of producing, by means of a solution of silver in conjunction with ferro-tartaric acid, a dormant picture brought out into a forcible negative impression by the breath, or moist air. The solution then described, and which had at that time been prepared some weeks, I may here accidentally remark, has retained its limpidity and photographic properties quite unimpaired during the whole year since elapsed, and is now as sensitive as ever—a property of no small value. Now, when a picture (for example, an impression from an engraving) is taken on paper washed with this solution, it shows no sign of a picture on its back, whether that on its face be developed or not ; but if, while the actinic influence is still fresh upon the face (i.e. as soon as it is removed from the light), the back be exposed for a very few seconds to sunshine, and then removed to a gloomy place, a positive picture, the exact complement of the negative one on the other side, though wanting of course in sharpness if the paper be thick, slowly and gradually makes its appearance there, and in half an hour requires considerable intensity. I ought to mention that the ferro-tartaric acid in question is prepared by precipitating the ferro-tartrate of ammonia by acetate of lead, and decomposing the precipitate by dilute sulphuric acid".